Fix reading qle_signedbitfield as int

The type-cast to int for qle_signedbitfield was wrong for all cases
where width + pos != 32. The class is currently only used two places
though, both where that happen to apply.

Change-Id: I108c565b75c9f29dd49b5e2e39f84910d17ead85
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/json/qjson_p.h

@@ -243,7 +243,7 @@ public:
         uint i = qFromLittleEndian(val);
         i <<= 32 - width - pos;
         int t = (int) i;
-        t >>= pos;
+        t >>= 32 - width;
         return t;
     }
     bool operator !() const {